Theorists often assume that individuals make choices by evaluating their alternatives on a common dimension or scale. Preferences are said to depend on this scale. This assumption of unidimensionality implies transitivity of preferences: If A is preferred to B, and B is preferred to C, then A must be preferred to C. Transitivity can exist in varying degrees of strength; for example, if A is selected over B 60 percent of the time and B over C 70 percent of the time, "weak" transitivity would prevail if A were chosen over C 55 percent of the time, but "strong" transitivity if A were chosen over C 80 percent of the time. Theoretically, weak transitivity implies that a single factor controls the direction of preferences, strong transitivity, that a single factor controls the exact choice probability. The present reseach applies this type of analysis to choices for "schedules of reinforcement," or work requirements. The objectives is to ascertain whether preferences for reinforcement schedules (which are often highly complex in nature) depend on a single dimension, as many theorists have suggested. If so, what is the nature of this dimension, and how does it relate quantitatively to choice? A second part of the research compares two widely-used procedures for studying choice behavior, one allowing an unlimited opportunity to make choices ("free-operant" procedure), and the other permitting choices only in "discrete trials. " With the alternatives held constant, does a subject show the same peferences under these two conditions? Or do the circumstances under which a subject makes choices, themselves affect preference? Theories of choice have traditionally related preferences to the values of stimuli and ignored the circumstances under which choices are made. The present research may show that this approach requires modification.